DESCRIPTION OF THE APPARATUS 73 



of light from the dark background must be prevented, 

 because, however feeble may be the light from this 

 source, since it affects the sensitized plate every time 

 the objective is uncovered, the ultimate sum of these 

 insignificant effects will finally become appreciable. 



One of the important factors in obtaining a good 

 negative is a pure atmosphere. Particles of (lust 

 floating in the air, when illuminated by the sun, form 

 a sort of luminous haze, which interferes with the 

 clearness of the photographic field. The effect is very 

 n diceable when a horse at a quick pace passes along 

 the track which stretches in front of the background. 

 The track should therefore be kept moist, the soil 

 in the neighbourhood should be turfed, and the inside 

 of the shed kept scrupulously clean. It often happens, 

 however, that the rays of the sun, although they do 

 not penetrate directly into the interior of the shed, 

 nevertheless impinge on the ground which surrounds 

 the entrance, and thus become reflected on to the velvet 

 curtain; the darkness of the photographic field is in 

 consequence considerably reduced. Even if the ground 

 of the shed is composed of asphalte, it is as well to 

 stretch a strip of velvet over those portions of it which 

 are directly illuminated by the sun.* 



In any case, the chances of light being reflected are 

 minimized by reducing the opening of the shed to the 

 smallest possible dimensions. It seldom happens that 

 the range of movement under observation equals the 

 whole length of the shed, which measures eleven metres 

 in this dimension. CKten, too, it is unnecessary to utilize 

 the entire height of the opening, which measures four 

 metres. This can be reduced by means of blinds and 



* An arrangement which would be perfect, but of which the resources 

 of the Physiological Station do not admit, would be to lower the level 

 of the ground inside the shed, so as to make it impossible for sunlight 

 to reach it. 



